(1) Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a novel bead portion-reinforcing structure directed at the improvement of the durability in bead portions of heavy duty pneumatic radial tires.
(2) Description of the Prior Art
In general, the heavy duty pneumatic radial tire comprises a main tire body reinforcement comprising a carcass composed of at least one ply containing metal cords arranged in the radial direction, a tread reinforcement comprising a belt composed mainly of at least two layers of metal cords superimposed about a crown portion of the carcass, the cords of which being crossed with each other at a small inclination angle with respect to the equatorial plane of the tire, and a bead portion reinforcement comprising a layer of metal cords obliquely arranged along that turnup portion of the carcass ply which is wound around a bead core in each of the bead portions from the inside to the outside with respect to the cords of the carcass turnup portion.
FIGS. 1a to 1c and FIGS. 2a to 2c illustrate conventional bead portion reinforcements, respectively, in which numeral 1 is a carcass ply, numeral 2 a turnup portion of the carcass ply, numeral 3 a bead core, numeral 4 a bead portion-reinforcing layer and numeral 5 a bead portion. FIGS. 1a to 1c each illustrate the case where the outer end of the bead portion-reinforcing layer 4 extends over the turnup portion 2, while FIGS. 2a to 2c each illustrate the case that the outer end of the bead portion-reinforcing layer 4 extends up to a position lower than the upper end of the turnup portion 2. On the other hand, the inner end of the bead portion reinforcing layer 4 is terminated at a position near a bead heel 6 in case of FIGS. 1a and 2a and at a position near a bead toe 7 in case of FIGS. 1b and 2b and is wound around the bead core 3 from the outside toward the inside in case of FIGS. 1c and 2c.
In any case, these bead portion reinforcements are designed to prevent the occurrence of separation failure at the end of the carcass turnup portion by restraining the outward deformation of the bead portion during running of the tire (i.e., enhancing the resistance to the deformation of the bead portion toward the rim flange under a load applied to the tire) and restraining the permanent strain of the bead core. However, any of the conventional bead portion reinforcements as shown in FIGS. 1a to 1c and FIGS. 2a to 2c have a limit to improve the durability of the bead portions.
That is, in order to restrain the outward deformation of the bead portion and the permanent strain of the bead core, it is preferable to increase the rigidity of the reinforcing layer and turn up the reinforcing layer around the bead core toward the inside of the tire (FIGS. 1c and 2c). But, with respect to the structure of FIG. 1 in which the outer end of the reinforcing layer 4 extends over the upper end of the carcass turnup portion 2, when the rigidity of the reinforcing layer is increased, the strain at the outer end of the reinforcing layer becomes larger than that at the upper end of the carcass turnup portion, so that the separation is unfavorably caused at the outer end of the reinforcing layer. Therefore, in the structure of FIG. 1, it is necessary to employ a reinforcing layer having a small rigidity, which accordingly imposes a limit on the improvement in the durability of the bead portion.
On the other hand, with respect to the structure of FIG. 2 in which the outer end of the reinforcing layer extends up to a position lower than the upper end of the carcass turnup portion, it is insufficient for the protection against the upper end of the carcass turnup portion although the durability of the bead portion is improved to a certain extent by the increase of the rigidity of the reinforcing layer and the turn-up structure of the reinforcing layer around the bead core (FIG. 2c), which has also a limit on the improvement in the durability of the bead portion.
Under the above circumstances, a demand exists for a more advantageous bead portion reinforcement which can simultaneously achieve the increase of the resistance to the outward deformation of the bead portion and the restraint of the permanent strain of the bead core and also can improve durability against separation failure at not only the upper end of the carcass turnup portion but also the outer end of the reinforcing layer.